If you touch it with your finger is it hard (coralline) or slimy (cyano).

If it turns out to be coralline this is desirable (except not on glass you wish to view through) just scrape it off with an algae scraper or credit card.

If it is cyanobacteria / blue-green algae thsi is not desirable. Usually shows up when tank is cyclign (i.e. in new set-ups) and is usually associated with high levels of dissolved organci carbon (DOC).